Exhaust gases emitted from internal-combustion engines of automobiles or the like that use gasoline as a fuel include harmful components such as hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxide (NOx). As such, it is necessary to purify each of the harmful components using an oxidation-reduction reaction before emission. For example, it is necessary to purify the exhaust gas such that the hydrocarbon (HC) is converted into water and carbon dioxide by oxidation, the carbon monoxide (CO) is converted into carbon dioxide by oxidation, and the nitrogen oxide (NOx) is converted into nitrogen by reduction.
As a catalyst for treating such an exhaust gas emitted from an internal-combustion engine (hereinafter, referred to as “exhaust gas purification catalyst”), a three-way catalyst (TWC) capable of oxidizing and reducing CO, HC, and NOx has been used.
As this type of three-way catalyst, a catalyst, in which precious metal is supported on a refractory oxide porous material such as an alumina porous material having a wide surface area, and then supported on a substrate such as a monolithic substrate made of a refractory ceramic or metallic honeycomb structure, or on refractory particles, is known.
The exhaust gas purification catalyst is required to always exert high purification performance even under the condition where an air-fuel ratio varies. Therefore, a promoter having oxygen storage/release capacity (OSC) (also referred to as “OSC material”) is allowed to coexist with precious metal, thereby securing purification performance.
An oxide such as ceria is the OSC material having oxygen storage/release capacity by which trivalent and tetravalent of Ce ions in a crystal lattice are reversibly changed. The OSC material that is allowed to coexist with precious metal is able to reduce a change in the exhaust gas atmosphere and to greatly improve the purification rate.
In the conventional OSC materials, that is, the oxygen storage/release materials, there has been a problem that the materials cannot exhibit oxygen storage/release performance at lower temperature when precious metal, which is a rare resource, does not exist therein.
Therefore, as an oxygen storage/release material not requiring precious metal, a stacked mixed layer irregular crystal structure delafossite-type oxide having specific oxygen storage/release characteristics is proposed (see Patent Document 1).
However, the above oxide has oxygen storage/release capacity only at a relatively high temperature. Therefore, a delafossite-type oxide, which does not require precious metal, having high oxygen storage/release capacity in a range of low temperature region to high temperature region, and a method for producing the same are proposed in Patent Document 2. Patent Document 2 discloses a delafossite-type oxide that is a 3R type delafossite-type oxide represented by a general formula ABOx (wherein A represents at least one element selected from the group consisting of Cu, Ag, Pd, and Pt, and B represents at least one element selected from the group consisting of Al, Cr, Ga, Fe, Mn, Co, Rh, Ni, In, La, Nd, Sm, Eu, Y, and Ti); and a method for producing the same, in which raw material powders are mixed in a desired composition ratio, press molded, and calcined under an inert atmosphere.
In addition to the above, in regard to the delafossite-type oxide, CuMO2 (M═Al, Cr, Fe) as a hexagonal system 2H delafossite-type oxide has been examined as an N2O decomposing catalyst (see Non-Patent Document 1).